System and method of operation of a digital mass flow controller

ABSTRACT

The mass flow controller of the present invention includes a sensor. This sensor is used to detect a mass flow within a gas line. Additionally, this sensor provides an output to an electronic control system coupled to the sensor. The electronic control system will determine an expected mass flow based on the output of the sensor. The electronic control system will adjust a control valve with a control signal to regulate a first gas flow through the control valve.

TECHNICAL FIELD OF THE INVENTION

[0001] This invention relates generally to systems and methods foroperating a mass flow controller (MFC) with a closed loop control systemutilizing an advanced digital control algorithm. More specifically, thepresent invention provides a closed loop control system for operating amass flow controller, wherein all mathematical operators are realizedwithin a digital processor.

BACKGROUND OF THE INVENTION

[0002] Many manufacturing processes require that the introduction ratesof process gases into a process chamber be strictly controlled. Thesetypes of processes use mass flow controllers (MFCs) to control the flowrate of gases. Many problems exist with current technology mass flowcontrollers.

[0003] MFC's have a high cost of ownership due to unscheduledmaintenance of the tools in which the MFC are installed. Often there isno indication as to a specific type of failure. Common practice is toreplace the MFC since it is perceived as a dynamic device that isbelieved to be inherently unreliable. As a result, a significant numberof MFC's are returned to the factory and found to operate as specifiedresulting in a no problem found failure analysis.

[0004] The lack of internal diagnostics or the ability to provide remoteservice of the MFC and remote diagnostics necessitates that a welltrained field service engineer or application engineer must visit thecustomer site to provide on-site tech support and failure analysis oncean MFC has been installed. Another shortcoming is that the individualdevice performance, specifically accuracy and response time or transientperformance, is dependent upon a time-consuming, labor-intensive, manualcalibration and tuning process, using potentiometers or variableresistors.

[0005] Today, the manufacturing process is a highly manual processrequiring that a technician use several devices such as oscilloscopes,different secondary flow measurement devices, and the like and visualinspections of those other devices to determine certain signals andtunes the potentiameters to his satisfaction.

[0006] This creates a set of devices that are dependent upon thepersonnel that tune the devices. The devices are commonly non-uniformand not repeatable from one unit to the other creating complex processengineering and characterization problems so each device has its ownunique behavior associated with the device. MFC behavior is directly afunction of the manual tuning process and the technician that performsthe process. Additionally, the transit response of traditional mass flowcontrollers are not uniform. The performance from, for example, 0 or 10%of set point is different from 0-100%. This varying response creates aproblem for the process control engineers that depend on these devices.The unique response of an individual device forces engineers to uniquelycharacterize MFC device behavior to account for the variability and theresponse times depending upon certain situations. This characterizationprocess is both expensive and time consuming.

[0007] Current MFC's are sensitive to inlet pressure. The mass flowcontroller requires a certain inlet pressure specification and thedifferential pressure is normally specified to be approximately 45 PSIDso the performance of existing MFC devices are typically optimized as toone specific pressure. Trade-offs are made when the inlet pressure isvaried over a particular pressure range, which creates more variabilityof the response time and characterization of a device.

[0008] One solution to inlet pressure sensitivity requires that the userinstall often expensive pressure regulators.

[0009] In the gas lines that supply the gas to the MFC it would beadvantageous to have a mass flow controller that is not sensitive topressure fluctuations in order that expensive pressure regulationhardware can be removed. Furthermore, each device has a pressuretransducer adjacent to the pressure regulator which sole purpose is toindicate that the pressure regulator is functioning.

[0010] Furthermore, there is a need to comply with emerging standardsopen communication standards and instrumentation. EIARS485 is an openstandard for multi-drop but it only describes the physical layer. So thesoftware protocol stack is subject proprietary implementations from onesupplier to another. So it is desirable to have mass flow controllersthat have high performance communications service ability that implementopen protocols while not sacrificing flow control parameters.

[0011] A powerful solution to understanding unknown or poorly understoodprocesses is to learn of the process through regression analysis.Regression analysis is a structured approach utilizing carefullydesigned experiments to optimize multivariable engineering processes.This technique allows a process to be understood and potentiallyexploited through a series of experiments. The usual method ofestimation for the regression model is ordinary least squares (OLS).

[0012] Regression analysis also allows the creation of diagnosticprocedures that compare the predicted values to actual values toevaluate the performance of the regression estimates through the use ofresiduals.

[0013] The overall task is to provide the best flow control performancepossible while not sacrificing other issues such as communications,multiple gas calibrations and cost of ownership.

SUMMARY OF THE INVENTION

[0014] The present invention provides a system and method for operatinga mass flow controller that substantially eliminates or reducesdisadvantages and problems associated with previously developed systemsand methods for operating a mass flow controller.

[0015] The chosen architecture of the present invention does notsacrifice to flow control for the ability to service a communicationsnetwork.

[0016] The present invention provides embedded diagnostics within thechosen architecture or system. Specifically, the digital engine of thepresent invention will discretely monitor system variables. Thesevariables include but are not limited to the flow set-point, solenoidcurrent, ambient temperature, the resistance of the flow sensor, and theinlet pressure which may be measured by an external pressure transducer.Several of these variables are a valuable source of information tomonitor and to reduce MFC pressure sensitivity. The digital engine alsowill monitor power supply voltages.

[0017] The internal digital mass flow controller embedded systemarchitecture specifically improves to improve the performance of an MFCover prior art systems and adds several value-added features.

[0018] The mass flow controller of the present invention does notcontain any variable manual adjustments. This provides an advantage inthat all calibration and tuning is completed digitally via storage of aunique set of constants stored in non-volatile memory. Access to therelevant memory locations is provided via the dedicated RS485 interface.The calibration system host, running the appropriate software andinterfacing to specific flow measuring instrumentation, canautomatically calibrate and tune the mass flow controller of the presentinvention for uniform repeatable performance in static and transientflow conditions. These uniform transient responses provide an additionadvantage of the present invention. These uniform transient responsesare achieved by using the computational power provided by the selecteddigital signal processor as applied in the chosen architecture. Thisarchitecture allows 100% of the control algorithms to be implemented viasoftware. The software algorithms can contain mechanisms to invokeunique case-sensitive or situational parameters that can be selected ortuned to achieve a uniform and repeatable transient response.

[0019] Yet another advantage of the present invention is to eliminatethe need for expensive upstream pressure regulators within a gas supply.The digital engine of the present invention has the capability todiscretely monitor the inlet pressure via an available A/D input. It isdesirable to monitor the input pressure and desensitize the mass flowcharacteristics of the output of the present invention.

[0020] A further advantage of the present invention is that it cancontain multiple autonomous dedicated digital communication ports. Thisarchitecture allows multiple digital networks to be servicedsimultaneously. Specific embodiments of the present invention providethat one network is an RS485 type network. The chosen DSP includes anembedded UART peripheral that is dedicated to service RS485 networks. Anadditional communication network can be serviced via reading and writingto a dual-port SRAM. The selection of the dual-port SRAM as acommunication partition enables the support of autonomousinterchangeable interfaces. The present invention should not benecessarily limited to these two communication ports. Multiplecommunication ports of various communication protocols as known to thoseskilled in the art can be incorporated into the present invention toachieve autonomous interchangeable interfaces.

[0021] An additional advantage of the present invention is that theembedded system manages events based upon receiving sample data from theflow signal at precise discrete intervals of 1.68 milliseconds. Due tothe computational power of the chosen architecture, the control'salgorithm completes its task in less than 30% of this time.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] A more complete understanding of the present invention and theadvantages thereof may be acquired by referring to the followingdescription, taken in conjunction with the accompanying drawings inwhich like reference numbers indicate like features and wherein:

[0023]FIG. 1 provides a block diagram for one embodiment of the presentinvention;

[0024]FIG. 2 illustrates a timing diagram of event management within thepresent invention;

[0025]FIG. 3 contains two sets of flow diagrams illustrating the methodof the present invention;

[0026]FIG. 4 describes memory interactions within the present invention;

[0027]FIG. 5 provides an overview of the communications associated withone embodiment of the present invention;

[0028]FIG. 6 describes a method and system for providing automatedcalibration of the mass flow controller of the present invention; and

[0029]FIG. 7 depicts an overview of diagnostic capabilities of thepresent invention;

DETAILED DESCRIPTION OF THE INVENTION

[0030] Preferred embodiments of the present invention are illustrated inthe FIGUREs, like numerals being used to refer to like and correspondingparts of various drawings.

[0031] The present invention provides an architecture system for adigital mass flow controller control system that includes embeddeddiagnostics within the control system. Specifically, the digital engineof the present invention will discretely monitor specific systemvariables for the purpose of improving troubleshooting and preventivemaintenance programs. These variables include but are not limited to theflow set-point, solenoid current, ambient temperature, the baseresistance of the flow sensor, the inlet pressure which may be measuredby an external pressure transducer. These variables are a valuablesource of information to monitor and to reduce MFC pressure sensitivity.The digital engine also will monitor power supply voltages.

[0032] The monitored variables are used to determine a more accurateflow measure within the MFC of the present invention.

[0033] The present invention uses a digital engine, meaning digitalsignal processor interfaced to an A-to-D converter. And specifically, inone embodiment of the present invention an architecture or a digitalsignal processor that has 16 channels of 10-bit A-to-D conversion hasbeen chosen. In this architecture, embedded A-to-D channels are used foracquiring the desired data. The present invention uses an externalintelligent A-to-D converter to digitize the sensor output since itrequires higher resolution than 10 bits. In addition to those monitoredparameters, several flow control events are monitored including thesteady to state error. Further, the accuracy of the flow controller withrespect to the set point provided by the user's tool is continuouslymonitored. The present invention also measures the valve leakage.

[0034] The present invention contains a unique algorithm executed by adigital engine. The specific algorithms will determine whether or notthe set-point for flow control is zero and if measured flow is greaterthan 2% of full scale which is an industry standard to provide a warningthat the valve is leaking. A common failure mode for the MFC is a failedactuator or valve. This failure manifests itself either as a leak orinability to control. Therefore, one would measure steady state error,valve leak and measure whether or not significant overshoot exists ofthe response at rising or ascending set points. For example, if oneobserves the set-point 2.0 from zero to 100% full scale and if theset-point exceeds or the response overshoots a predetermined value, thena warning is provided to the user. Providing access to all of thesemonitored parameters will reduce down time and the unnecessary removalof MFC devices otherwise properly functioning. The architecture of thepresent invention minimizes the time required to service and repair MFCdevices that are not functioning properly because of the level ofdiagnostics provided to localize the full analysis process to specificareas.

[0035] It is desirable that open communication networks, such asDevicenet™, and the like, will be available for the flow control processmanagement system to control the device. Another communications port isdedicated to service. Therefore, an RS485 port or other like port isdedicated to providing service. Therefore, communication is establishedvia the open communications port for process control. There is adedicated RS485 network that has a proprietary communications protocolthat is compliant with the OSI model for networking within the presentinvention.

[0036] An OSI model for networking is an open system. The OSI model is amethod that is commonly used to network PC's. This model is a low-level,broad model for implementing open systems interconnects for PC's.

[0037] The dedicated factory service port enables the present inventionto provide remote diagnostics. The port can be used to implement localmonitoring when linked to a dedicated laptop notebook or PC. Networkmonitoring via the port can be implemented by interfacing a laptopnotebook or PC to the RS485 interface and outfitting or integrating amodem into the PC. Therefore an engineer, with modem access, can dialinto the local computer and interrogate the present inventiontransparent to the flow control events. Further, the embeddeddiagnostics may be utilized by this engineer to provide a higher levelof detail concerning potential failure modes.

[0038] The FIGUREs support the problems addressed by the presentinvention, as well as illustrating why the present invention'sarchitecture provides advantages and features that are a function ofsolving the problems associated with the prior art.

[0039]FIG. 1 provides a block diagram for one embodiment of the presentinvention's embedded system or architecture. Block 12 represents theflow sensor. An A-to-D converter 14 monitors flow sensor 12. A-to-Dconverter 14 is interfaced to a 16-bit microcontroller or digital signalprocessor (DSP) 16.

[0040] One embodiment of the present invention utilizes a TexasInstrument TMS320F240 DSP as DSP 16. This DSP contains embedded flashmemory that allows the software code within the DSP to be upgraded froman external network connection.

[0041] DSP 16 stores internal algorithms or software that can beexecuted to implement a closed loop control system. DSP 16 provides anelectronic signal 72 to valve drive module 18. Valve drive module 18 inturn provides current to solenoid 20. Solenoid 20 serves as the actuatormechanism to control valve 22.

[0042] Block 24 provides a voltage reference to the A-to-D converter 14.A-to-D converter 14 is embedded in flow controller or DSP 16.

[0043] There are two types of A-to-D converters:

[0044] (1) those embedded in flow controller or DSP 16, and

[0045] (2) an external instrumentation class A-to-D converter. Adjacentto the voltage reference is a serial electronically erasableprogrammable read-only memory (EEPROM) or non-volatile RAM (NVRAM) 26.EEPROM 26 is used to store unique calibration and gas-specific data withserial number, service data and various diagnostic codes. Diagnosticsare one of the major features that the architecture of the presentinvention brings forth. To the right of the DSP 16 is a bi-color LEDcircuit 28 and a static ram or SRAM 30. In addition to SRAM 30 isexternal dual port ram 32 (DPRAM). DRAM 32 is connected tocommunications co-processor 34. Communications co-processor 34 isinterfaced to a sensor bus network with transceiver 36. This sensor busnetwork may be a DeviceNet, LonWorks, Profibus, ethernet or other likenetwork as known to those skilled in the art.

[0046] An additional networking interface is provided via RS485transceiver 38 embedded within flow controller or DSP 16.

[0047] The above blocks represent the major functional blocks locatedwithin the embedded system of one embodiment of the present invention.

[0048] Various networks may interface with the present invention. Theembodiment above illustrates two network connections. However, thepresent invention need not be limited to two connections and may containadditional network connections. Multi-drop network 40 may interface withRS485 port 38. Multi-drop networks 40 and DeviceNet network 42 areexamples of potential network connections. Multi-drop network 40requires local PC 44 to act as a host. Local PC host 44 may contain amodem connection 48 allowing the present invention to be accessed viaanother modem 48 located or interfaced to remote PC 50.

[0049] DeviceNet network 42 can either be a peer-to-peer or peer-to-hosttype network. If the network has a peer-to-host relationship withrespect to the DeviceNet network 42, the PC host can also monitor thenetwork. This PC host may be PC 44 but need not be monitored by PC 44.Hence, this potentially separate PC is denoted as PC 52. This particularPC host 52 can also interface to a modem 54. Modem 54 can link via atelecom connection that can link remote modem 48 and associated remotePC 50.

[0050] Several signals and associated path warps are contained withinthis embodiment of the present invention. First, flow sensor 12 isexcited with a current source 13 generating approximately 11 milliamps.Using thermal or heat transfer laws, as known to those skilled in theart, the mass flow rate is sensed. Signal 15, generated by flow sensor12, is monitored by an A-to-D converter 14. This A-to-D converter isintelligent. Not only does A-to-D converter 14 monitor the flow signal;A-to-D converter 14 is programmable with respect to the resolution andsampling frequency.

[0051] The sampling frequency chosen for one embodiment is 610 Hz. 610Hz provides a sampling period of 1.6 milliseconds. It is important tonote that A-to-D converter 14 has a data ready signal which interruptsthe microcontroller to let it know that it has a full set of sampledata. This is important because of a multi-tasking embedded system thatserves several input/output functions. The data ready signal interruptsthe entire software, minimizing the effect of time delays that createinaccurate flow samples. The data ready signal issued provides the maintrigger for the main functional software block, or software engine. Nexta data signal may be sent over A-to-D converter 14 serial interface 56which may be referred to as an SPI™ TM interface.

[0052] Via serial interface 56, flow controller 16 monitors A-to-Dconverter 14 and acquires a sample representative of the measured flowsignal 15. This is repeated each time that the data ready signalindicates that A-to-D converter 14 has a new sample. In one embodiment,A-to-D converter 14 does this precisely at 1.6 millisecond intervals.However, it should be noted this interval is programmed and thereforeflexible. 1.6 milliseconds was chosen for this embodiment because flowsensor 12 has an effective bandwidth of approximately 100 to 120 Hz.Therefore, 610 Hz was chosen as an effective sampling frequency toappropriately reconstruct the flow signal due to the analog-to-digitalconversion process. Additionally, 610 Hz was chosen to provide enoughtime to serve other functions in a way that does not interrupt the datasignal or interfere with other serviced events. This allows a mastersignal to be used as a trigger, leaving time to service other events.

[0053] On the left-hand side of flow controller 16, several externalsignals or analog signals 58-68 are monitored or interfaced to fromeither an internal or external source to the present invention. Anexample would be an analog set-point 58. Set-point 58 is a request fromthe host by the user for the intended flow rate. Typically, thisset-point 58 is provided by a user's wafer processing tool when thepresent invention is used in a semiconductor processing environment.Additionally, digital 5-volt signal 60, +15 volt power supply 62, −15volt power supply 64, a user's inlet pressure 66 and internal ambienttemperature 68 inside the case of the mass flow controller are monitoredby DSP 16.

[0054] The signal for internal ambient temperature 68 may be provided asan auxiliary function by the voltage reference device 24. Therefore,voltage reference device 24 may provide a precision 2.5-volt referenceto A-to-D converter 14 in addition to a temperature output. Oneembodiment of voltage reference device 24 has an embedded band gapreference and temperature-assisted component or resistor on board, and atransfer function that relates resistance to temperature in degreesCelsius. Therefore, a measurement at room temperature in conjunctionwith the transfer function provides a real-time reading of the internalcase temperature.

[0055] On line 72, valve drive circuitry 18, conditions signal 72 todrive a solenoid, but it also feeds back the actual solenoid current 70for sensing purposes. The sensed solenoid current 72, set-point signal55 and the measured flow 15 can be used to create relationships thatprovide faster diagnosis of problems.

[0056] A Universal Asynchronous Receiver Transmitter (UART) is used togenerate a half duplex using an interface to an external host such as aPC RS485 network. This UART circuit does several things. The circuitprovides access to all of the calibration and tuning parameters duringmanufacturing. The UART also provides another means for a user tocontrol the MFC of the present invention.

[0057]FIG. 2 illustrates a timing diagram of event management. UART offlow controller 16 is coupled to RS485 with a transmit line 74 andreceive line 76. TX indicates a transmitted signal and RX indicates areceived signal from the host system of the flow controller and DSP 16.

[0058] A 9600 baud half duplex polled protocol line couples the RS485XCVR 38 to multidrop network 40. 9600 baud was chosen in a mannersimilar to the sampling frequency. This provides an interface to anexternal host. This speed allows a safety margin with respect to otherevents that are serviced while not interfering with such other events.Several events inside the flow controller are monitored to providediagnostics. Local indication of the status of those events is providedvia a bi-color LED circuit 28 that can either be red or green. If LEDcircuit 28 is flashing red, then a monitored condition or event is abovea warning condition or a warning threshold. If LED circuit 28 is solidred, then the warning has become an alarm. In most cases, an alarmcondition indicates that the flow controller has experienced anunrecoverable fault.

[0059] The flow controller, DSP 16, will continue to attempt to operate.The event that caused the unrecoverable condition to occur is saved innonvolatile memory. Therefore, the event can be accessed via RS485transceiver 38.

[0060] A current limiting resistor provides a feedback path frombi-color LED circuit 28 to a second IO port on the flow controller 16.The color of bi-color LED circuit 28 is controlled by the direction ofthe current flow through LED circuit 28 provided by the two IO portsfrom DSP 16.

[0061] An embedded pulse width modulator (PWM) is utilized to constructindicated flow signal 80. The same type of circuit is also used toreconstruct the sensed solenoid current 82.

[0062] DSP 16 has an external memory interface. The external memoryinterface interfaces to SRAM 30. This SRAM is utilized during theboot-up process and receives the contents of a serial EEPROM 32 forfaster access.

[0063] Transceiver 36 can go to either a DeviceNet network 42 or may beconnected through some other type of network such as LonWorks, Profibus,Ethernet or others known by those skilled in the art.

[0064] In addition to serving existing drop-in applications, which areanalog, a future standard digital protocol, or several competingprotocols can be served by the architecture of the present invention.This allows full compatibility as choices in networking technology andprotocols are realized. The present invention provides an autonomousinterface such that the flow controller system would not be affected bymodular compatibility in a modular scheme. The DPRAM provides a digitalpartition between the DSP 16 and a to-be-determined sensor BUS network84.

[0065] SRAM 30 is not exclusively external. A portion or all of SRAM 30may be incorporated as internal SRAM into DSP 16 or the digital flowcontroller of the present invention.

[0066] In the illustrated embodiment, the SRAM is divided into twoportions, external SRAM 30, and internal SRAM inside the flash memory ofDSP 16. Using SRAM inside DSP 16 of digital flow controller allows allcontrol functions to be executed within DSP 16.

[0067] Key attributes of the architecture described above eliminate thereliance on manually adjusted potentiometers. This is achieved in thepresent invention by choosing an architecture that has sufficientdigital bandwidth to monitor flow sensors, generate control signals,such as joint differentiators and proportional interval controllers, andstore multiple calibration constants while providing access to suchconstants in real time. Next, the digital flow controller does notdepend upon multiple potentiometers to adjust the control system, ratherthe present invention depends upon several variables declared asconstants. Access to these variables is provided via the RS485communications port 38.

[0068] Digital communications services, such as servicing an RS485 hostor a DeviceNet host, do not affect flow control events. The DPRAM 32allows service on a scheduled basis.

[0069] Several points are monitored for diagnostics. Visual feedback isprovided to a user via bi-color LED circuit 28 which has two levels ofindication, either warning or an alarm, flashing or solid. This sameinformation is available from the RS485 port and may be presented by asoftware application executed within an PC in a GUI interface. Controlsystem functions are implemented via software. The present inventionprovides advantages over previous technology that utilized analogcircuits, which are sensitive to aging effects and temperature, toconstruct the closed-loop control system.

[0070] The architecture of the present invention allows all of theexecutable embedded software to be programmed via the RS485 port. Newcode can be installed without removing the mass flow controller. It isimportant to note that flash memory embedded in DSP 16 is not used tostore any calibration constants or unique data for the flow controller.This data is stored in an external serial EEPROM. The flow controlsystem of the present invention provides autonomy and compatibility withexisting and to-be-determined communication protocols such as DeviceNet.

[0071]FIG. 2 relates to technical advantages of event management withinthe architecture of the present invention. FIG. 2 illustrates the eventtime line executed by the A-to-D converter 14 of FIG. 1. When A-to-Dconverter 14 has acquired data and is ready to send the acquired datavia the data BUS to DSP 16, a ready signal is sent.

[0072] All data processing takes place within time frame A-B, once thedata has been dumped from A/D converter 14. This is the amount of timethat is taken by the control system for all data processing. Betweenevents B and C, enough time is left for other tasks to be accomplished.DeviceNet interrupt 112, real-time interrupt 114, and RS-485 interrupt116, as illustrated in FIG. 2, occur between events B and C. If theseinterrupts occur between events A and B, processing collected data stillhas priority. Hence, these interrupts are still processed between eventsB and C.

[0073]FIG. 3 contains two sets of flow diagrams. FIG. 3 furtherdescribes and details the interrupts previously shown in the timingdiagram provided in FIG. 2. A method of the present invention begins atstep 100. All initializations of DSP 16 and peripherals occur at step101. During the initialization step 101 of the DSP and relatedperipherals, A-to-D converter 14 will be programmed to sample at aspecific frequency. In one embodiment of the current invention, thisfrequency is chosen at 610 Hz. This 610 Hz is a signal to startacquiring data samples from A-to-D converter 14 and is the trigger forthe external ADC interrupt to step 105. At step 103, the software codeof the present invention continuously loops and awaits one of fourinterrupts. These interrupts include the external ADC interrupt whichoccurs every 1.68 milliseconds, the real-time interrupt which occursevery 1.04 seconds, a DeviceNet interrupt, and an RS485 interrupt.External ADC interrupt 105 allows all data processing to occur duringthe servicing of the ADC interrupt.

[0074] In one embodiment of the present invention, data processing takesapproximately 30% of the total time between ADC interrupts, 1.68milliseconds. During this time, at step 102, the flow controller willacquire all ADC-related data from A-to-D converter 14 of FIG. 1. Next,at step 104, this data is linearized. One such method for linearizationis detailed in U.S. patent application Ser. No. ______ entitled “Systemand Method for Sensor Response Linearization,” filed ______ by ThomasPattantyus, et. al. In this linearization process, previously determinedcoefficients are utilized to relate acquired ADC data to a more accurateflow value. At step 106, linearized data and values are sent to thecontrol system loop which will manipulate the mass flow controller toachieve the desired mass flow rate.

[0075] Step 108 involves the bi-colored LED circuit 28 of FIG. 1.Depending on the state of the flow controller 16, the LED circuit 28would light appropriately according to a signal and current directionsupplied to the bi-colored LED circuit 28 at step 108.

[0076] At step 110, diagnostics are performed. These diagnostics aredirectly related to the displayed state of the bi-colored LED circuit28. DeviceNet handling occurs at step 112. At step 114, data is selectedor retrieved from serial EEPROM 26. This data may include calibrationconstants, gas-specific data, serial numbers of the mass flow controllerand related controllers, service data and diagnostic codes. At step 116,data is acquired from the RS485 interface. At step 118, the totalizertotals or integrates the time and flow measurements inside the flowcontroller 16. At step 120, miscellaneous housekeeping functions mayoccur, after which a return from this priority interrupt may occur.

[0077] Following the return from the external ADC interrupt 105, theprocess will return to the beginning step 100.

[0078] Any additional data sensed which would cause any additional orother interrupts, such as the real-time interrupt 126, the DeviceNetinterrupt 122, or the RS485 interrupt 130, are held during the externalADC interrupt 105. External ADC interrupt 105 takes priority over allother data processing. During the processing time of approximately 50milliseconds, data associated with other interrupts is stored in SRAM 30or the internal RAM located within DSP 16. No matter which interruptprocess occurs, when the interrupt process is complete, the method ofthe present invention will return to step 103 and await a futureinterrupt.

[0079] Real-time interrupt 124, DeviceNet interrupt 126, and RS485interrupt 130 all allow data to be read and written to variables andstored within the DSP 16. Minimum operations occur during this interruptto handle this data. The actual processing of this data will happen whenit is accessed during the external ADC interrupt 105. An additional wayto think of this is that these interrupts provide and object-orientedsystem linked through memory locations. Therefore, when an interruptoccurs, data is stored in the appropriate memory location until theexternal ADC interrupt 105 occurs and the data is processed.

[0080]FIG. 4 describes memory interactions within the present invention.This figure describes how the SRAM 30, EEPROM 26, and DPRAM 32 interact.For purposes of this diagram, no distinction is made for data RAM orSRAM 30 between internal RAM located on the flash memory of DSP 16 orexternal SRAM 30 accessible to DSP 16. SRAM 30 in this case merelydescribes a memory location in which to store data. Duringinitialization, all EEPROM data is copied into the SRAM 30 as indicatedby link 142. Additionally, the top 1K of EEPROM data is copied to DPRAM32 during initialization via link 144.

[0081] An additional link 146 occurs between EEPROM 26 and SRAM 30,during which critical data is copied to the SRAM 30 duringinitialization. This second link is shown because copying initializationdatA-to-DPRAM 32 is a two-step process. EEPROM data cannot be copieddirectly to DPRAM 32. Therefore, EEPROM data is first copied into alocation within SRAM 30. From SRAM 30, this data is then copied to DPRAM32 via link 144. Critical data copied to SRAM 30 during initializationwould contain control system data, which goes to the internal memorylocation, and data for totalizers and the like.

[0082] A third link 150 indicates data being transferred from SRAM 30 toEEPROM 26. This link allows totalizer and diagnostic data to betransferred during DSP 16 operation. Link 152 is a two-way link betweenSRAM 30 and RS485 communication port 38 that allows data to read fromand write to SRAM 30.

[0083] A second communication pathway exists between EEPROM 26 and RS485communication port 38 via link 154, which allows all data to be read andwritten to EEPROM 26 from RS485 communication port 38. Therefore, thepresent invention allows full access to both EEPROM 26 and SRAM 30 data.

[0084] The present invention allows all data within SRAM 30 and EEPROM26 to be accessed via RS485 communication port 38. However, a user maybe limited from full access to EEPROM or SRAM data. This may be done forsecurity purposes to ensure the unique serial number and other data fromthe vendor cannot be changed by the user. One example may be associatedwith warranty data or serial numbers of individual components.

[0085] The present invention provides an object-oriented approach wherethere is a method and interface from one object to another to enablediagnostics to be performed or data to be requested from or passed toanother object. Certain prior technologies may have an RS485 protocolthat use memory locations embedded in the protocol. This priortechnology solution has a problem in that if the wrong data istransmitted to a specified memory location, a device will tend tomalfunction and behave outside its specified performance. The presentinvention is built to perform tests to determine if requested datashould be provided, to protect the data that should not be madeaccessible, and to test data before it is processed to ensure that it isappropriate.

[0086] Data transfer occurs via link 154, because EEPROM 26 isnon-volatile, crucial data can be saved here for DSP 16. EEPROM 26 willretain data when power is lost to DSP 16. Additionally, this data may beaccessed via RS485 communication port 38. Other data saved here would begas-specific calibration data. The configuration of each mass flowcontroller may be dependent upon the mechanical configuration.Therefore, link 154 may be used from RS485 communication port 38 to gaindirect access to EEPROM 26 to download the majority of those parametersin a fashion that does not require unique selection and tuning in orderto accelerate the manufacturing process. Therefore, in the manufacturingprocess unique selection and tuning is not required, enhancing the speedof this process. When the device is initialized, EEPROM 26 is blank. Thecalibration tables and other such data may be downloaded en masse viaRS485 communications link 156 to a multi-drop network 40. Multi-dropnetwork 40 again is linked to a PC host 44.

[0087] The present invention is also designed to be compatible withlegacy systems. This backward compatibility allows users to communicateto an embodiment of the present invention with an analog signal. Theanalog communications 160 as shown provide a functional link to eithermonitor a flow signal or provide a set-point control to a legacy system,as known to those skilled in the art. These analog communications may berealized by A-to-D converter functions located within A-to-D converter14. These analog communications 162 occur between analog communicationssystem 160 and a user host for legacy host system.

[0088] DeviceNet interface module 42 is linked to DPRAM 32. Acommunications co-processor may send an interrupt to the DPRAM inquiringas to its availability to receive data. Every data transfer will be aninterrupt except for static data such as an ID number. Frequentlyaccessed data will be stored in DPRAM 32 to avoid the interrupt asopposed to accessing EEPROM 26. This increases the overall efficiency byavoiding interrupts by having the readily accessible data in DPRAM 32.

[0089]FIG. 5 provides an overview of the communications associated withone embodiment of the present invention. FIG. 5 illustrates and providesan overview of three types of communication protocols which may becoupled to or compatible with one embodiment of the present invention,the DeviceNet, analog, or RS485 protocols. The present invention shouldnot be limited to these types of protocols or communications systems,rather any combination of these communications systems. It is importantto note that the architecture of the present invention can accept analogset-points from legacy systems and interface with different sources. Itmay also accept digital set-points and information via DeviceNet asknown to those skilled in the art, or a proprietary RS485 system. Thisis accomplished within one common architecture.

[0090] Within each communications source, for example with the RS485interface, it is important to note that the device may have a servicemode and a normal operating mode and potentially any number ofadditional modes. The normal operating mode will have limited access.The service mode or command mode will have the capability to allow anauthorized provider to have full access to the device, whereas thesecommands and systems may not be available or necessary in the normaloperating mode for the normal user.

[0091] This systems or command mode allows access to diagnosticinformation. In the embodiment of the present invention shown in FIG. 5,calibration and tuning is done through the RS485 port and is notaccessible via the DeviceNet port. The DeviceNet port is used forprocess control only. In addition to process control, the DeviceNet portmay provide access to diagnostics that are in a DeviceNet language,alarms or exceptions.

[0092] In the analog mode, output flow values and valve voltage may beprovided. Analog systems basically provide a linear op-amp circuit thatprovides the voltage present on the solenoid. The present inventionadvances the art by precisely controlling the current deliveredproviding a direct relationship with the force required to manipulatevalve position.

[0093] Another advantage associated with the present invention is thatthe present invention is non-intrusive. Data need merely be monitoredand analyzed. Using predictive algorithms, it is possible to save onmanufacturing and maintenance costs by developing a predictivemaintenance system for regular maintenance. Additionally, it is possibleto determine potential failures associated with the system. Thisanalysis may identify operation outside of a normal range but that isstill within specification. This may indicate abnormal operation or thata trend or other analysis may indicate a potential failure modeassociated with the system. Therefore, a sensor degradation or valvemalfunction can be determined and replaced before serious damage occursto the host system.

[0094] Trend analysis and relational data are analyzed within adiagnostic system. One embodiment of the present invention does not makedecisions in the embedded system as to whether or not the system shouldoperate, but rather the present invention develops a warning or alarmcondition. The present invention may be coupled to an external systemcontaining a graphical user interface on a PC or a similar softwareanalysis packages known to those skilled in the art, to develop trendand relationships among different data and improve the value ofdiagnostics.

[0095] In FIG. 1, local PC host 44 may be the host of a multi-dropnetwork which can be connected to a modem 46. The objective is toprovide remote access to data from the present invention. FIG. 1 depictsin functional terms a second modem 48 in a remote PC 50 where remoteaccess of the data occurs. However, the application or transport layerof the network could consist of a TCPIP or other internet application.Modem 46 and modem 48 indicate just one means of realizing a transportlayer for the networking capability to provide remote access to datafrom the present invention. The 9600 baud half duplex mode protocol usedby the present invention does comply with the seven-layer OSI model fornetworking, which makes it compatible with a TCPIP stack should onechoose to network the present invention over an internet connection.

[0096]FIG. 6 describes a method and system for providing automatedcalibration of the mass flow controller of the present invention. A massflow controller of the present invention is not calibrated, as priorsolutions were, by making mechanical adjustments. Rather, a set ofstored coefficients is altered to calibrate the accuracy of the massflow controller of the present invention. Calibration consists mainly oftwo steps. One, linearization. This step basically irons out thenon-linear behavior of the sensor and provides a linear value for thedigital flow controller 16. The set-up depicted in FIG. 7 is used wherethe calibrator provides either an analog or digital set-point. For everyapplied set-point, a corresponding sensor and controlled “flow standard”value is recorded. A regression technique, as known by those skilled inthe art, is applied to these points. Once calculated, these coefficientsare recorded in EEPROM 26 of the present invention for use. The task ofthe present invention then becomes a numerical exercise where apolynomial is used with the determined coefficients to calculate thecorrect flow from sensed variables. This linearization process may berepeated for individual gases and stored inside EEPROM or other memorylocation within the present invention.

[0097] Dynamic tuning provides the ability to measure flow duringtransient events. Logic will be incorporated into the host PC that willadjust the set-point conditions in order to control flow conditions andachieve the expected flow in a transient condition.

[0098] An additional embodiment of the present invention may incorporatea system to record actual flow conditions and sensed flow conditions.This data may be analyzed statistically for various mechanical platformsto which the mass flow controller of the present invention may beattached. This data can be used to establish trends and, in conjunctionwith logic or artificial intelligence as known to those skilled in theart. Control parameters can be incremented for a given performancecriteria until such performance is optimized.

[0099]FIG. 7 provides a overview of the diagnostic capabilities of thepresent invention. Bi-color LED circuit 28 provides a status indicator.LED circuit 28 indicates when green that the present invention isoperational. If the bi-color LED circuit 28 is flashing red, a minorrecoverable fault is indicated. If bi-color LED circuit 28 indicates asolid red, a unrecoverable fault has occurred. A final condition of thebi-color LED circuit 28 is no light, which indicates that there is nopower to the MFC of the present invention. The status of bi-color LEDcircuit 28 is determined after a software code within the mass flowcontroller evaluates whether monitored variables are operating withinspecified parameters. These variables include outputs from the sensor,solenoid valve, the ambient temperature of the device of the presentinvention, the power supply to the present invention, and a measure ofdynamic performance of the present invention. Any unrecoverable faultsthat are determined by the software code will be stored within theEEPROM non-volatile memory 26. Additionally, all data values can be madeavailable over the DeviceNet port or RS485 port 38. The data from RS485port 38 or the DeviceNet port can be sensed by a software diagnosticprogram running on a portable computer such as a laptop or otherspecialized computing device. This software would provide a user aninterface that would give the user a detailed view of the results of thediagnostic routines running inside the present invention. A specializedtool such as a hand-held diagnostic tool, can also sense softwarediagnostic data from RS485 port 38 or specified test points within thepresent invention. For the DeviceNet port, a separate and uniqueinterpreting mechanism is incorporated to read the results of thesoftware diagnostics. A second software program running on a portablecomputer or portable diagnostic tool can be configured in an additionalembodiment to read the results of the software diagnostics from theDeviceNet port.

[0100] The present invention addresses many problems associated with theprior art. High cost of ownership assumed by a user due to unscheduledand unexpected failed mass flow controllers that are often arbitrarilyreplaced without any justification. This is because historically massflow controllers have been seen as unreliable and undependable, and manyusers do not understand how a mass flow controller operates. Because ofthis failure to understand the operation of the mass flow controller andthe problems associated with calibrating a mass flow controller, fieldservice is required to provide technical support and failure analysis ofsuspected mass flow controllers once installed on a system.Historically, mass flow controllers have been tuned or calibrated basedon an individual technician's explicit knowledge of device performance,using multi-turn potentiometers or variable resistors. This calibrationis a time- and labor-consuming subjective event directly affectingdevice performance. Because of the manual tuning and calibration of themass flow controllers of the prior art, non-uniform transient responseis often experienced with their use. Furthermore, the mass flowcontrollers of the prior art have been found to be extremely sensitiveto inlet pressure. Additionally, the mass flow controllers of the priorart are generally analog systems or proprietary RS485 networks.

[0101] The present invention provides a system containing the abilityfor forward compatibility with multiple open communications standardssuch as DeviceNet, LonWorks, and Profibus which still maintaining abackwards compatibility with legacy interfaces.

[0102] Troubleshooting and analysis of failed mass flow controllers haverequired that the mass flow controller be removed from operation andinstallation within its installed system. This is a costly event can beeliminated by directly monitoring the mass flow controller in operationas is accomplished by the present invention. The present inventioncontains a digital engine which discretely monitors the set-point,solenoid current, ambient temperature, sensor base resistance, and inletpressure indication, as well as power supply voltages, flow errors,valve leak and transient response. These monitored parameters can beanalyzed (manually or automatically) to track and determine abnormaltrends associated with the operation of the device of the presentinvention. Furthermore, this data can be monitored remotely via a modemor other network connection to a host PC.

[0103] The mass flow controller of the present invention does notcontain any variable manual adjustments. This provides an advantage inthat all calibration and tuning is completed digitally via storage of aunique set of constants stored in non-volatile memory. Access to therelevant memory locations is provided via the dedicated RS485 interface.The calibration system host running the appropriate software along withinterfacing to specific flow measuring instrumentation can automaticallycalibrate and tune the mass flow controller of the present invention foruniform repeatable performance in static and transient flow conditions.These uniform transient responses provide an addition advantage of thepresent invention. These uniform transient responses are achieved byusing the computational power provided by the selected digital signalprocessor as applied in the chosen architecture. This architectureallows 100% of the control algorithms to be implemented via software.The software algorithms can contain mechanisms to invoke uniquecase-sensitive or situational parameters that can be selected or tunedto achieve a uniform and repeatable transient response.

[0104] Yet another advantage of the present invention is to eliminatethe need for expensive upstream pressure regulators within a gas supply.The digital engine of the present invention has the capability todiscretely monitor the inlet pressure via an available A-to-D input. Itis desirable to monitor the input pressure and desensitize the mass flowcharacteristics of the output of the present invention.

[0105] A further advantage of the present invention is that it containsmultiple autonomous dedicated digital communication ports. Thearchitecture of the embodiment described above allows two digitalnetworks to be serviced simultaneously, provided that one of them is anRS485 type network. The chosen DSP includes an embedded UART peripheralwhich is dedicated to service RS485 networks. An additionalcommunication network can be serviced via reading and writing to adual-port SRAM. The selection of the dual-port SRAM as a communicationpartition enables the support of multiple autonomous interchangeableinterfaces. The present invention should not be necessarily limited tothese two communication ports. Multiple communication ports of variouscommunication protocols as known to those skilled in the art can beincorporated into the present invention to achieve multiple autonomousinterchangeable interfaces.

[0106] An additional advantage of the present invention is that theembedded system manages events based upon receiving sample data from theflow signal at precise discrete intervals of 1.68 milliseconds. Due tothe computational power of the chosen architecture, the control'salgorithm completes its task in less than 30% of this time.

[0107] The present invention provides historical data, trends, analysis,relational data. Such data can be archived enhancing a servicetechnicians ability to remotely diagnose or locally diagnose via a hostsystem connected to a dedicated service port to determine currentoperation.

[0108] Various mass flow controllers exist on the market. A mass flowcontroller may include a linearization circuit to aid in linearizing theflow sense signal. Particular reference is made to U.S. patentapplication Ser. No. ______ filed on Jul. 9, 1999, by T. T. Pattantyus,et al., entitled “System and Method for Sensor Response Linearization.”A mass flow controller may also include an improved Mass Flow InterfaceCircuit that measures mass flow within a mass flow controller by sensingthe resistance change of a sense resistor or resistors in response togas flow. Particular reference is made to the improved mass flowinterface circuit disclosed in U.S. patent application Ser. No. ______filed on Jul. 9, 1999, by D. S. Larson et al., entitled “Improved MassFlow Sensor Interface Circuit.” A mass flow controller may also includea derivative controller that corrects the sensed flow signal to moreaccurately approximate the actual flow through the mass flow controller.Particular reference is made to the derivative controller disclosed inU.S. patent application Ser. No. ______ filed on Jul. 9, 1999, by E.Vyers, entitled “System and Method for a Digital Mass Flow Controller.”A PI controller can also be included in a mass flow controller togenerate a valve drive signal to control a valve in the mass flowcontroller. The PI controller can increase the speed of response of themass flow controller and compensate for a nonlinear response of thevalve to the valve drive signal. Particular reference is made to the PIcontroller disclosed in U.S. patent application Ser. No. ______ filed onJul. 9, 1999, by E. Vyers, entitled “System and Method for a VariableGain Proportional-Integral (PI) Controller.” Lastly, the valve drivesignal in a mass flow controller can be input to valve drive circuitryto control a solenoid activated valve. Reference is made to the valvedrive circuitry disclosed in U.S. patent application Ser. No. ______filed on Jul. 9, 1999, by T. T. Pattantyus, entitled “Method and Systemfor Driving a Solenoid.” It is important to note that the presentinvention is not limited to use in a mass flow controller that includesthe components described above.

[0109] Although the present invention has been described in detailherein with reference to the illustrative embodiments, it should beunderstood that the description is by way of example only and is not tobe construed in a limiting sense. It is to be further understood,therefore, that numerous changes in the details of the embodiments ofthis invention and additional embodiments of this invention will beapparent to, and may be made by, persons of ordinary skill in the arthaving reference to this description. It is contemplated that all suchchanges and additional embodiments are within the spirit and true scopeof this invention as claimed below.

What is claimed is:
 1. A mass flow controller comprising: a sensor; anelectronic control system coupled to an output of the sensor; a controlvalve which receives a control signal from the electronic control systemwherein the control signal is used to regulate a first gas flow throughthe control valve; and a bypass valve which creates a pressure drop andmaintains a constant ratio of a flow through the sensor and the betweenthe sensor and the gas flow through the mass flow controller.
 2. Themass flow controller of claim 1, further comprising an embeddeddiagnostic system, wherein the diagnostic system identifies fault orpotential fault conditions in the mass flow controller.
 3. The mass flowcontroller of claim 2, wherein the embedded diagnostic system provides avisual indication of fault or potential fault conditions in the massflow controller.
 4. The mass flow controller of claim 2, wherein theembedded diagnostic system further comprises an interface to communicatefault or potential fault conditions in the mass flow controller toremote diagnostic system.
 5. The mass flow controller of claim 4,wherein the remote diagnostic system includes a historical database ofmass flow controller fault conditions and indications of these faultconditions.
 6. The mass flow controller of claim 1, wherein theelectronic control system utilizes a control algorithm to provide adesired output flow through the mass flow controller independent ofpressure and flow fluctuations within a supply flow to the mass flowcontroller.
 7. The mass flow controller of claim 1, wherein electroniccontrol system: calculates a real time flow error using an algorithmwhich receives an input on a series of system variables comprising: adesired output flow setpoint; a solenoid current; an ambienttemperature; a base resistance of the sensor; an inlet pressureindication; at least one power supply voltage; a leakage through thecontrol valve; an overshoot of actual output flow as compared to thedesired output flow setpoint; and adjusts the control signal to thecontrol valve to achieve the desired output flow based on a real timecalculated flow error.
 8. The mass flow controller of claim 7, whereinthe algorithm is derived is a polynomial expression using regressiontechniques, wherein the expression is stored as a series of constants ina memory location accessible by the electronic control system.
 9. Themass flow controller of claim 8, further comprising: a calibrationsystem to monitor the series of system variables and the actual flowoutput and calculate the series of constants representing the polynomialexpression.
 10. The mass flow controller of claim 9, further comprising:at least one data communication port coupled to the electronic controlsystem and operable to communicate data between the electronic controlsystem and at least one external network.
 11. The mass flow controllerof claim 9, wherein the at least one data communication port supportsperphials chosen from the group of RS485 or UART.
 12. A method fordetermining the flow of mass through a mass flow controller comprisingthe steps of: sensing flow through a sensor and outputting the sensedflow to an electronic control system; maintaining a constant ratiobetween the flow through a sensor and the flow through the mass flowcontroller; calculating an actual real time flow through the mass flowcontroller within the electronic control system; determining a flowerror between a desired flow setpoint and the actual real time flow; andgenerating a control signal within the electronic control system whichis operable to adjust the position of a control valve within the massflow controller; throttling the flow through the mass flow controller tominimize the flow error.
 13. The method of claim 12 wherein the step ofcalculating the actual real flow through the mass flow controllerutilizes an algorithm which receives input on a series of variables. 14.The method of claim 13 wherein the algorithm is determined usingregression analysis and represented by a series of constants whichrepresent factor effects for the input variables.
 15. A method ofcalibrating a mass flow controller comprising the steps of: measuringthe real time actual flow through the mass flow controller with a flowmeasuring instrumentation system; sensing a series of system variablesassociated with a mass flow controller wherein the variables comprise: adesired output flow setpoint; a solenoid current; an ambienttemperature; a base resistance of the sensor; an inlet pressureindication; at least one power supply voltage; a leakage through thecontrol valve; a real time flow error between an actual output flow ascompared to the desired output flow set-point; modeling a predicted flowthrough the mass flow controller with a regression analysis technique toproduce a multivariable response function describing a response of themass flow controller to the system variables; and inputting themultivariable response function into an electronic control systemoperable to regulate the flow through the mass flow controller.
 16. Themethod of claim 20 further comprising the steps of: archiving themultivariable response function in a memory location within theelectronic control system.